The objective of this proposal is to use monoclonal antibodies to identify and characterize specific surface molecules of murine hematopoietic cells. The ultimate objective is to describe the pathways of differentiation in the hematopoietic system in terms of the gain and loss of specific surface molecules. Cell hybridization techniques will be used to produce a library of monoclonal antibody reagents defining individual cell surface molecules. Analysis of the monoclonal antibodies by indirect binding of radio-labelled antibody and by direct cytotoxicity on a panel of tumor cell lines will be used to identify reagents which are specific for molecules on particular classes of cells. Further experiments in which monoclonal antibodies are used to deplete or enrich normal populations of hematopoietic cells for cells bearing the molecule in question followed by assay of the population for specific classes of hematopoietic cells will be carried out to identity molecules which are specific for stem cells or progenitor cells. Emphasis will be placed on identification of molecules which are present on tumor cell lines known to respond to inducing compounds or which enable us to classify tumor cells whose normal cell counterpart is presently unknown. Selected antibodies will then be used in genetic and biochemical experiments to study the function, biosynthesis and regulation of the molecules which they identify. These studies should increase our knowledge of molecular mechanisms and cellular relationships in normal and abnormal hematopoietic differentiation.